KIC 3868420: A high-amplitude $\delta$ Scuti-$\gamma$ Dor hybrid star crossing the Hertzsprung gap

TL;DR

This study analyzes KIC 3868420, a rare high-amplitude hybrid star crossing the Hertzsprung gap, using four years of Kepler data and stellar modeling to reveal its pulsation modes and evolutionary status.

Contribution

It provides the first detailed asteroseismic analysis of a high-amplitude hybrid star in the Hertzsprung gap, demonstrating the coexistence of pulsation modes in a rapidly evolving star.

Findings

Detected 36 significant frequencies, including 11 independent modes.

Modeling suggests the star is a post-main-sequence with mixed p- and g-modes.

Highlights the rarity and importance of such hybrid pulsators in stellar evolution.

Abstract

We report a photometric and asteroseismic analysis of KIC 3868420, a newly identified high-amplitude $\delta$ Scuti-$\gamma$ Doradus hybrid star located in the Hertzsprung gap - a short-lived and rarely observed post-main-sequence phase. Using four years of Kepler long-cadence photometry, we detect 36 significant frequencies, including 11 independent modes spanning both low- and high-frequency regimes. Grid-based modeling with MESA and GYRE, including rotation, shows that five independent frequencies match a combination of radial p-modes and non-radial g-modes, supporting its hybrid nature. The best-fit models yield an evolved post-main-sequence star ($M \sim 2.26 - 2.30 M_\odot$, $R \sim 4.41 - 4.43 R_\odot$, $\tau \sim 5.4 \times 10^{8}$ yr), although degeneracies from rotation and unknown inclination preclude a unique solution. KIC 3868420 thus represents a rare example of a high-amplitude hybrid pulsator in the Hertzsprung gap, demonstrating that high-amplitude pulsation can coexist with mixed p/g-mode behavior in a rapidly evolving star. This finding highlights the value of space-based photometry for identifying such rare objects.